A hydraulic shock absorber for a vehicle comprises, for example, two fluid chambers separated in the interior of a cylinder by a piston, and a passage provided through the piston to connect these fluid chambers. A damping valve in the form of a leaf valve is provided at an outlet of the passage to generate a damping force relative to displacement of the piston. The leaf valve generally comprises a plurality of stacked leaves having a fixed inner circumferential part, and lifts an outer circumferential part to open the passage according to the differential pressure between the upstream and downstream sides of the leaves. With this construction, the damping force generated by the leaf valve tends to be excessive when in a middle to high speed region of piston displacement.
In order to improve the damping force characteristic of a leaf valve, JPH09-291961A, published by the Japan Patent Office in 1997, proposes a leaf valve in which the inner circumferential part is not fixed but supported resiliently by a coil spring.
Referring to FIG. 10, in a shock absorber in which this leaf valve is installed, a cylindrical piston nut N is secured onto a tip of a piston rod R penetrating the piston P. The leaf valve L closing an outlet of a passage Po which passes through the piston P is fitted on the outer circumference of the piston nut N such that it can displace in an axial direction. A coil spring S an end of which is supported by the piston nut N, resiliently supports the inner circumferential part of the leaf valve L via a push member M.
When the piston P moves upward in the figure, working oil in an oil chamber above the piston P flows into an oil chamber below the piston P via the passage Po and a damping force is generated due to a flow resistance of the leaf valve L at the outlet of the passage Po. When the piston displacement speed is in a low speed region, the leaf valve L bends the outer circumferential part downward in the figure from the inner circumferential part supported by the push member M. As the piston displacement speed reaches a middle to high speed region, the pressure in the passage Po becomes greater than the resilient force of the coil spring S such that the leaf valve L retreats from the piston P downward in an axial direction together with the push member M. As a result, the opening area of the leaf valve L becomes large so that the damping force is prevented from becoming excessive. As shown in FIG. 11, the damping force increase is gradual with respect to an increase in the piston displacement speed even in the middle to high speed region.